1. Field of the Invention
The present invention relates to a check valve, and more particularly to a check valve provided with a spring seat portion which has its spiral seat surface brought into press-contact with at least one of opposite terminal turns of a compression coil spring to support the compression coil spring.
2. Description of the Related Art
In general, as viewed from FIG. 3, a conventional check valve has a single cylindrical body element 21, slidably and coaxially mounted in an upstream-side portion of which body element 21 is a cylindrical valve element 23. This cylindrical valve element 23 has its inner shoulder portion brought into press-contact with one of opposite open ends of a compression coil spring 22 and is therefore constantly biased leftward to its closed position by a biasing device such as the compression coil spring 22 in a longitudinal direction of the cylindrical body element 21. Fixedly and coaxially mounted in a downstream-side bore portion of the cylindrical body element 21 is an annular stopper element 24 for receiving the other of the opposite open ends of the compression coil spring 22.
In operation, when the cylindrical valve element 23 is subjected to a fluid pressure which is larger than a predetermined value, the cylindrical valve element 23 is forcibly moved rightward by the fluid pressure against the biasing force exerted by the compression coil spring 22, so that a fluid passage of the check valve having been closed by the cylindrical valve element 23 is now opened. An O-ring 25 is fixedly mounted in an annular groove formed in an outer peripheral surface of a head portion of the cylindrical valve element 23 to ensure that the fluid passage of the check valve is fluid-tightly closed by the cylindrical valve element 23 when the fluid pressure is smaller than the predetermined value.
As for the biasing force exerted by the compression coil spring 22, it is desirable to have such biasing force applied evenly to the entire circumferential area of the inner shoulder portion of the cylindrical valve element 23. More specifically, when the biasing force is applied unevenly to the inner shoulder portion of the cylindrical valve element 23, the cylindrical valve element 23 tends to incline inside the cylindrical body element 21. Such inclination of the cylindrical valve element 23 inside the cylindrical body element 21 results in leakage of the fluid inside the cylindrical body element 21 in its closed configuration. In order to prevent such leakage of the fluid in the closed position of the cylindrical valve element 23, it is necessary to have each of the opposite open ends of the compression coil spring 22 formed into a closed-end flat shape extending in a vertical plane perpendicular to the longitudinal direction of the cylindrical body element 21, as viewed in FIG. 3. The thus formed closed-end flat shape of each of the opposite ends of the compression coil spring 22 is capable of being brought into surface-contact with each of the cylindrical valve element 23 and the stopper element 24, and therefore capable of having the biasing force of the compression coil spring 22 evenly applied to the entire circumferential area of the inner shoulder portion of each of the cylindrical valve element 23 and the stopper element 24.
In the art of semiconductor fabrication in which the check valve is used in transferring a cleaning fluid, the compression coil spring 22 serving as a biasing device of the check valve is coated with a tubular element made of fluororesins to improve the compression coil spring 22 in chemical resistance. Due to the presence of such tubular element, in the art of semiconductor fabrication, it is impossible to form each of opposite open ends of the compression coil spring into a closed-end shape.
As is clear from the above description, the compression coil spring 22 tends to obliquely push the cylindrical valve element 23 inside the cylindrical body element 21, which makes it impossible for the head portion of the cylindrical valve element 23 to be uniformly brought into press-contact with the O-ring 25. This results in leakage of the fluid in the area of the O-ring 25 when the check valve is in its closed configuration, and also results in vibrations and issuance of noise of the cylindrical valve element 23 inside the cylindrical body element 21 due to misalignment in longitudinal axis between the cylindrical valve element and the cylindrical body element 21.